Arrival and progression of the Swallow Hirundo rustica through Britain

Arrival dates of Swallows from 20 carefully chosen years of the phenological reports of the Royal Meteorological Society (1883-1947) were abstracted. These years represent observations from 1000 different localities in Britain and from between 24 and 325 localities per year. Arrival dates of the Swallow were processed in two different ways: (1) using a Geographical Information System, contour maps were produced for each year and compared subjectively; (2) using climatic data from Britain and Europe, between-year variation in average arrival date and in speed of progression through Britain were analysed statistically. Swallows progressed through Britain at an average speed of about 50 km day -1 . In 'early' arrival years, Swallows progressed along a southwest-northeast axis, entering Britain by the southwest (the region receiving the most accumulated heat by April). They travelled faster across Britain in 'late' arrival years along a south-north axis. Arrival dates were earlier during warmer Aprils in Britain and western France, and following a warmer March in Iberia. Although the timing of spring migration of the Swallow is related to European temperatures, the possible future effects of climatic changes may not be predictable because environmental conditions in Africa and changes in population size must also be taken into consideration.